<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
</head>
<body>
    <div id="test">hello world</div>
    <script>
    /* if语句中（）的值，接收boolean类型，如果不是boolean，会强转为boolean */
//    var age=20;
//    if(age){
//        alert("能上网")
//    }
   
      var test=document.getElementsByClassName("test")[0];
      console.log(test);
      if(test){
          test.style.color="red"
      }else{
          console.log("test没有被获取")
      }

    </script>

</body>
</html>